At present, the most widely used technologies for measuring the mechanical properties of materials at a high strain rate in the field of material science are split Hopkinson pressure bar technology and tension bar technology. The basic principle of this method is that a short sample is placed between two tension bars or pressure bars, tension stress waves or compression stress waves are input to an incident bar in a certain way to load the sample. Meanwhile, pulse signals are recorded by strain gages which are pasted on the tension bars or pressure bars and have a certain distance away from one end of each bar. If the tension bars or pressure bars remain in an elastic state, the pulses in the bars will be propagated undistortedly at the elastic wave speed. Thus the strain gages pasted on the tension bars or pressure bars may measure the change of the load acting on the ends of the bars over time.
For a Hopkinson pressure bar, a common way to generate incident waves is to shoot an impact bar at high speed through an air gun and generate incident pulses by coaxial collision with the incident bar. This method has the disadvantages that the installation locations of the impact bar in the air gun are different in each shoot and it is difficult to determine the correlation between impact speed and air pressure; therefore, it is not able to accurately control the amplitude of the incident waves and it is required to do many experiments to get a desired strain rate. Secondly, for an experiment with an oversized span of strain rate, there is a need to change the length of the impact bar to obtain different strain rates due to the limitation of the air pressure of the air gun; the higher the strain rate is, the shorter the impact bar is needed, and the shorter the stress waves in the experiment are; this limits the range of strain and makes the experiment complex. What's more, as there is a lower limit to the shooting speed of the impact bar, some lower strain rates (e.g. a strain rate of 10 s−1) in the experiment cannot be obtained by a traditional Hopkinson pressure bar. Because different experimental systems have different parameters, it is an international difficult problem to standardize the experimental technology of the split Hopkinson pressure bar.
For a Hopkinson tension bar, the commonly used loading method is that the impact bar of the tension bar is made into a hollow tube; the impact tube is shot at a high speed by the air gun; when the impact tube moves to the end of an incident bar, the impact tube collides with the lug boss on the incident bar end and generates a series of compression waves which are propagated to the lug boss end of the incident bar and reflected by the free end as tension waves; the sample is loaded by the tension waves through the incident bar. However, this loading method has many disadvantages: 1. the impact bar is shot from one end of the incident bar to the other end, so the incident bar is in an unsupported free state at the section from the lug boss on the incident bar to the air gun, and thus the incident bar is easy to bend; 2. this design limits the length of the impact tube to be about 500 mm, so the wavelength of the generated incident waves is about 0.2 ms, but for a ductile material and a low strain rate experiment, incident waves with a longer wavelength are required; 3. the impact tube is very inconvenient to replace; and 4. due to the limitation of the wall thickness of the impact tube, a very high air pressure is needed to accelerate the impact tube. Many scholars have proposed different design ideas: 1. add a lug boss at one end of the impact tube to increase the shooting speed of the impact tube, but the waveform generated by this method is affected by the lug boss and no longer regular; 2. use a hollow incident bar to allow the impact bar to pass through the incident bar, and this makes it difficult to reshape the waveform.
Because of the different shapes of the impact bar and the different positions of the air gun, the traditional Hopkinson pressure bar and tension bar loading systems cannot be realized on the same device.
In 1960s, in order to solve the problem of normal riveting, Boeing designated HuberASchmitt et al. to take the lead to research electromagnetic riveting technology, and a patent for a strong impact electromagnetic riveting apparatus was applied in 1968. Low-voltage electromagnetic riveting technology is developed successfully by Zieve Peter in 1986, which solves the problems in the riveting quality, promotion and application of high-voltage riveting technology, thus making electromagnetic riveting technology develop rapidly. Electromagnetic riveting technology has been applied in the manufacturing of Boeing and Airbus series aircraft. Today, low-voltage electromagnetic riveting technology has developed and become mature, and the magnitude and duration of riveting force may be controlled accurately. The technical principle of electromagnetic riveting is that a coil and a stress wave amplifier are added between a discharge coil and a workpiece. When a discharge switch is switched off, a primary coil generates a strong magnetic field around the coil by the rapidly changing impact current. A secondary coil coupled with the primary coil generates an induced current under the action of the strong magnetic field, and then generates an eddy current magnetic field; the two magnetic fields interact to generate an eddy current repulsion force which is transferred to a rivet through an amplifier to shape the rivet. The eddy current force has a very high frequency and is transmitted in the form of stress waves in the amplifier and the rivet, so electromagnetic riveting is also called stress wave riveting. If the principle of an electromagnetic riveting gun is applied to the split Hopkinson pressure bar to replace the air gun and impact bar in the traditional split Hopkinson pressure bar and generate stress waves by electromagnetic repulsion directly, it will be possible to standardize the experimental technology of the split Hopkinson pressure bar. In addition, the pulse width of the stress waves generated by electromagnetic induction may be adjusted by circuit parameters, and the pulse width may reach millisecond level, so some low strain rates lower than 102/s) which cannot be loaded by the traditional Hopkinson bar may be loaded. In the patents with application numbers of 201420098605.4 and 201410161610.X, a device solution and experiment method where an electromagnetic riveting apparatus is directly applied to a Hopkinson pressure bar device is proposed; however, the waveform obtained by this method has limitations; in the inventions with application numbers of 201410173843:1 and 201410171963.8 respectively, two experiment devices and use methods which may be applied to both a Hopkinson tension bar and a Hopkinson pressure bar are proposed, however, the structures of the two solutions are relatively complex and traditional waveform shaping technology cannot be applied to tension. In the invention with application number of 201510051071, a primary coil structure and use method for electromagnetic stress wave generator is proposed to increase the variation range of the amplitude and pulse width of the waves generated by the electromagnetic stress wave generator.